Transportable antibiotic resistance is a recently discovered phenomenon with significance for both basic molecular biology and for clinical microbiology. Many different antibiotic resistances, encoded on DNA elements called "transposons", and originally isolated on multiple resistance plasmids (R-factors) are found to transpose to new genomes, including different R-factors, virus genomes, and the bacterial chromosome. I propose to investigate the mechanism of this exotic process and also to utilize transposition mutagenesis to probe the genetic structure of virulence-associated plasmids in pathogenic bacteria. The basic rationale of the proposed approach is to utilize the coliphage lambda as a vector for physiological, genetic, and biochemical studies of the properties of transposons. Debilitated lambda vectors will be constructed for each of the major transposons and studies carried out to quantitate transposition to the bacterial chromosome. These vectors and a lysis-defective transposon insertion lambda mutant that I isolated will be used to search for transposon and bacterial mutations affecting transposition. Lambda DNA will be used as a vector for attempting to achieve transposition events in vitro, with the goal of directly investigating the biochemical mechanism and purifying the functions involved. Finally, lambda and other vectors will be used in transposition mutagenesis of virulence plasmids of Escherichia coli and Yersinia enterocolitica, in order to identify and map the genes responsible for the virulence.